1. Field of the Invention
The present invention generally relates to a servo unit. More particularly, the invention relates to an electromagnetic servo unit of the type in which input torque is electromagnetically servo-controlled for output.
2. Description of Relevant Art
Most conventional servo systems, exemplarily of those applied to power steering systems for vehicles, are constituted mainly using a hydraulic mechanism.
Such conventional hydraulic servo system, as applied to a power steering system for vehicles, have hydraulic oil from a hydraulic pump sent under pressure into a servoactuator such as a hydraulic power cylinder or hydraulic servomotor for servo control of input torque from a steering wheel. The servo control provides a controlled output for an operating element to be actuated, in accordance with the operational conditions of the steering wheel.
However, one problem in such a hydraulic servo system is that a hydraulic pump is normally put into service, whether or not an effective input is given from a steering wheel. Thus, power is uselessly consumed. An additional problem is that many component parts are employed such as an actuator, hydraulic pump, control valve, and oil reservoir. These parts occupy a relatively large space as a whole, which presents a problem in attaining a compact, lightweight design and reduced cost.
To overcome the problems of a conventional hydraulic servo system, a number of electromagnetic servo systems have already been proposed. In the U.S. Patent Publication No. 2,754,465 an electromagnetic servo system is applied to a power steering system for vehicles. According to this U.S. Patent, the electromagnetic servo system includes a direct-current motor coaxially arranged on a steering shaft interposed between a steering wheel as an input member of the power steering system and a gear as an output member thereof. A torque sensor of a well-known strain guage type is attached to the steering shaft. The torque sensor detects torque applied from the steering wheel to the steering shaft, to thereby control the direct-current motor.
Though having effectively solved conventional problems attendent the hydraulic servo system to a certain extent, the strain gauge type torque sensor in the electromagnetic servo system according to the U.S. Patent, is not provided with sufficient protection, and is susceptable to disturbance such as dust, moisture, and external forces.
Moreover, in this electromagnetic servo system, in addition to the strain gauge type torque sensor, also provided is a limit switch for detecting the axial travel limit of an input shaft of the electromagnetic servo system. The limit switch is connected to the steering wheel side thereof which is axially slidable. The limit switch is arranged on the vehicle body side at the side of a stationary member. Also provided is an amplifier with a power unit. The amplifier receives signals from the torque sensor and the limit switch, and is used for controlling the output torque of the direct-current motor. As a result of having so many parts, the entire system is complicated. Further, an operation control circuit of the direct-current motor is remarkably complicated as well.
Furthermore, in this servo system in which the input shaft is axially slidable as mentioned above, a drive section thereof including the input shaft, and output shaft, and the direct-current motor has a variable longitudinal dimension during operation. Thus, an additional provision of a complicated fixing mechanism is needed when applying the servo system to a certain mechanism or system such as a power steering system for vehicles.
On the other hand, later on, there has been proposed another electromagnetic servo system in Japanese Patent Lay-Open Print No. 58-141963 laid open on Aug. 23, 1983. The electromagnetic servo system according to the Japanese Lay-Open Print, is applied to a power steering system for vehicles. The servo system includes an electromagnetic motor coaxially arranged on a pinion shaft of the power steering system of a rack and pinion type which gives the pinion shaft an additional torque. This servo system as well has effectively solved the aforementioned conventional problems of the hydraulic servo system to a certain extent.
However, also in the electromagnetic servo system according to the Japanese Laid-Open Print, a torque detection and control mechanism are separately arranged, with respect to a drive section thereof including the electromagnetic motor. The torque detector is for detecting torque applied to the pinion shaft as an input side member. The control mechanism receives a detection signal from the detector to thereby control the output torque of the motor. As a result, like the case of the aforementioned U.S. Patent, the servo system is susceptive to disturbances and, is complicated.
Further, in the servo system according to the Japanese Lay-Open Print, the electromagnetic motor is controlled to always have torque output in proportion to the steering torque as applied to the pinion shaft from a steering wheel, thus being inherently bound to unnecessary consumption of power.
The present invention effectively solves the problems of a conventional electromagnetic servo system, in a further improved form.